Does Hydroxyzine Cause Diarrhea or Constipation?

Hydroxyzine is considerably more likely to cause constipation than diarrhea. As a first-generation antihistamine with notable anticholinergic activity, it tends to slow gut motility rather than speed it up, and constipation appears in clinical data as one of its recognized gastrointestinal side effects. Diarrhea is not a commonly reported adverse event with hydroxyzine, though it can occur in certain individuals and circumstances. The distinction comes down to how the drug interacts with receptors in the digestive tract, and understanding that mechanism helps explain why some people experience one, the other, or neither.

How Hydroxyzine Affects the Digestive Tract

Hydroxyzine works primarily by blocking histamine H1 receptors, which is what makes it useful for allergic reactions, itching, and anxiety. But histamine receptors are not just sitting in the skin and brain waiting to cause hives and sneezing. They are distributed throughout the entire gastrointestinal tract, from the stomach to the colon. H1 and H2 receptors appear on the nerve cells that coordinate gut movement, on the cells lining the intestinal wall, and within the muscular layers that push food along. Histamine acting on these receptors helps regulate gastric acid production, intestinal motility, and fluid secretion across the gut lining.¹

When hydroxyzine blocks H1 receptors in the gut, it dampens some of the signaling that promotes intestinal contractions and fluid secretion. On its own, that can slow things down. But hydroxyzine does not stop at histamine receptors. It also has anticholinergic properties, meaning it partially blocks acetylcholine, a neurotransmitter that is one of the gut’s primary “move things along” signals. Acetylcholine stimulates the smooth muscle contractions that push digested food through the intestines. Blocking it reduces those contractions. This is why dry mouth, another classic anticholinergic effect, commonly shows up alongside constipation in hydroxyzine users: both are consequences of the same receptor-blocking action.

Constipation Is the More Commonly Reported Effect

In a real-world observational study of 400 patients taking hydroxyzine hydrochloride for chronic itching, only about 3% reported any adverse events at all. The side effects that did appear were mild to moderate and included dizziness, drowsiness, dry mouth, sedation, and constipation. Constipation was reported by roughly half a percent of patients in that study, the same frequency as drowsiness and dry mouth.²

Those numbers sound reassuringly low, and for most people they are. But observational studies of this kind tend to capture only side effects that patients spontaneously mention to their clinician, which means milder or embarrassing symptoms like constipation are probably underreported. If you are taking hydroxyzine and notice your bowel habits slowing down, the anticholinergic mechanism offers a clear explanation. The drug is reducing the muscular contractions and fluid secretion your gut relies on to keep things moving.

The constipation is typically dose-dependent. Higher doses of hydroxyzine mean stronger anticholinergic effects, and the gut feels the impact. Hydroxyzine is prescribed across a wide range of doses, from 10 mg for mild anxiety or itching up to 100 mg or more per day in divided doses. Someone on 25 mg at bedtime for sleep or anxiety is less likely to notice digestive slowdown than someone taking 50 mg three times daily for severe urticaria.

Can Hydroxyzine Cause Diarrhea?

Diarrhea is not listed among the commonly reported side effects of hydroxyzine in most clinical literature, and the drug’s pharmacology makes constipation the expected direction. That said, some people do report loose stools or diarrhea while taking it, and there are a few plausible reasons this can happen.

Individual variation in gut sensitivity is one factor. Some people have highly reactive intestinal tracts that respond unpredictably to new medications, even ones that pharmacologically should slow motility. The inactive ingredients in hydroxyzine tablets or syrup, such as certain fillers or flavorings, can occasionally cause gastrointestinal upset in sensitive individuals. The liquid formulation of hydroxyzine, in particular, contains ingredients that some people find mildly irritating to the stomach.

Another scenario involves the body adjusting to the medication. In the first few days of starting hydroxyzine, some people experience transient GI symptoms that settle once the body acclimates. This is common across many drug classes and is not unique to antihistamines.

A third and more clinically interesting possibility involves the rebound effect when hydroxyzine is stopped. If you have been taking hydroxyzine regularly and then discontinue it abruptly, the gut’s histamine receptors, which have been chronically blocked, may become temporarily more responsive. This can trigger a burst of increased motility and fluid secretion, leading to loose stools. This rebound phenomenon is better documented with H2 blockers used for acid reflux, but it is theoretically relevant to any antihistamine that has been suppressing gut receptor activity for an extended period.

When Multiple Drugs Amplify the Problem

Hydroxyzine’s anticholinergic effect on the gut does not exist in isolation. Many commonly prescribed medications also have anticholinergic properties, and when they stack up, the combined effect on the bowel can be much more significant than any single drug would produce alone. This concept, sometimes called anticholinergic burden, matters especially for people taking several medications at once.

Older adults are particularly vulnerable. They are more likely to be on multiple prescriptions, and their bodies are less efficient at clearing anticholinergic drugs. Research on anticholinergic side effects in older patients has highlighted that what starts as mild constipation from one drug can escalate to fecal impaction when a second or third anticholinergic agent is added.³

Common drugs with anticholinergic activity that might be combined with hydroxyzine include certain antidepressants (tricyclics like amitriptyline, and some SSRIs to a lesser degree), bladder medications like oxybutynin, muscle relaxants, and other first-generation antihistamines like diphenhydramine. If you are taking hydroxyzine alongside any of these, the cumulative gut-slowing effect is greater than any one drug’s label would suggest. Flagging the combination to your prescriber is worth doing, especially if constipation becomes persistent or uncomfortable.

How Hydroxyzine Compares to Other Antihistamines

Not all antihistamines carry the same risk of GI side effects. A laboratory study that ranked ten common H1 antihistamines by their anticholinergic potency found hydroxyzine in the middle of the pack for in vitro activity, below cyproheptadine, promethazine, desloratadine, diphenhydramine, and loratadine, but above pyrilamine. The picture shifted in live testing, where hydroxyzine did not show significant anticholinergic activity at the doses tested, while cyproheptadine, promethazine, desloratadine, loratadine, and diphenhydramine did. Two newer-generation antihistamines, cetirizine and fexofenadine, showed no anticholinergic activity at all in either model.⁴

This is a useful detail if you are taking hydroxyzine primarily for allergies or itching and finding that constipation is bothering you. Cetirizine, which happens to be the active metabolite your liver produces when it processes hydroxyzine, has essentially no anticholinergic properties. Switching to cetirizine can preserve much of the antihistamine benefit while eliminating the gut-slowing side effect. Fexofenadine is another option with a clean anticholinergic profile. The trade-off is that these second-generation drugs do not cross into the brain as readily, so they lack hydroxyzine’s sedative and anxiety-reducing effects. If you are taking hydroxyzine specifically for anxiety or as a sleep aid, a straight swap to cetirizine will not cover those uses.

Diphenhydramine, the active ingredient in many over-the-counter sleep aids and allergy pills, is notably more anticholinergic than hydroxyzine in laboratory and animal testing. If constipation is your concern, switching from diphenhydramine to hydroxyzine might actually be a step in the right direction, though both will still carry more GI risk than the second-generation alternatives.

Practical Steps if You Notice Changes

If you start hydroxyzine and your bowel habits shift toward constipation, a few straightforward adjustments can help before you consider changing the medication itself:

Increase fluids: Anticholinergic drugs reduce fluid secretion into the gut, so drinking more water partially compensates.
Add fiber gradually: Soluble fiber from foods like oats, beans, and fruit helps draw water into the stool. Overdoing it without enough water can make constipation worse, so ramp up slowly.
Timing matters: If you take hydroxyzine at bedtime for sleep, the anticholinergic effects peak while your gut is already in its slowest overnight mode. Some people find that taking it earlier in the evening gives the gut a little more active time before the drug peaks.
Watch for stacking: Review your other medications and over-the-counter products for additional anticholinergic effects. Even common cold remedies and sleep aids can add up.

If these adjustments are not enough and constipation is persistent, discussing a dose reduction or a switch to a less anticholinergic antihistamine with your prescriber is reasonable. Do not discontinue hydroxyzine abruptly if you have been using it regularly for anxiety, as your prescriber may want to taper to avoid symptom recurrence.

Mast Cells, Gut Disorders, and a Counterintuitive Twist

There is an interesting wrinkle for people who have conditions involving overactive mast cells, the immune cells that release histamine. In conditions like irritable bowel syndrome, researchers have found increased numbers of mast cells in multiple regions of the intestine, including the small bowel, the area where the small and large intestines meet, and the colon.⁵ These extra mast cells release excessive histamine locally, which can drive diarrhea by ramping up fluid secretion and gut motility through those very H1 and H2 receptors lining the intestinal wall.

For these individuals, hydroxyzine may actually improve diarrhea rather than cause it. By blocking the H1 receptors that the excess histamine is overstimulating, hydroxyzine can reduce the heightened motility and secretion that are producing loose stools. This is not a common use of the drug, and it is not a first-line treatment for IBS or diarrhea, but it illustrates why blanket statements about a drug’s GI effects can be misleading. The same pharmacological action that causes constipation in a person with a normally functioning gut can relieve diarrhea in someone whose gut is flooded with histamine.

A related condition, mast cell activation syndrome, involves mast cells releasing bursts of histamine and other mediators throughout the body, often triggering wide-ranging symptoms including GI distress. In case reports, patients treated with hydroxyzine alongside other antihistamines have seen broad symptom improvement, including resolution of GI complaints that had resisted other treatments.⁶ These are small case series rather than large trials, so the evidence is still emerging. But for someone with unexplained diarrhea and a suspected mast cell disorder, the fact that hydroxyzine suppresses histamine-driven gut overactivity is genuinely useful information.

Why GI Side Effects Vary So Much Between People

One of the most frustrating aspects of GI side effects is how unpredictable they are from person to person. Two people can take the same 25 mg dose of hydroxyzine, and one develops noticeable constipation while the other feels no change at all. Several factors explain the variation.

Liver metabolism plays a role. Hydroxyzine is processed by enzymes in the liver, and the speed of that processing varies genetically. People who metabolize hydroxyzine slowly end up with higher blood levels for longer, which means more sustained anticholinergic activity in the gut. Fast metabolizers clear the drug more quickly and may experience fewer side effects at the same dose.

Baseline gut function matters too. If you tend toward constipation already, whether from diet, other medications, or a naturally slower transit time, hydroxyzine’s additional slowing effect is more noticeable and more likely to push you into symptomatic territory. Conversely, someone with a faster baseline transit time has more margin before the drug’s effects become apparent.

Diet and hydration interact with the drug’s anticholinergic properties in direct, mechanical ways. A diet low in fiber and fluids offers the gut less bulk and lubrication to work with, so any additional reduction in motility and secretion from hydroxyzine hits harder. This is one reason the constipation side effect tends to be more prominent in people who are already sedentary, eating a low-fiber diet, or not drinking much water, a profile that overlaps heavily with the older adult population where anticholinergic burden is already a concern.

The gut microbiome adds another layer of complexity. Histamine is not just a human signaling molecule; certain gut bacteria produce it too. The local histamine environment in your intestines is partly shaped by which bacterial species dominate your microbiome. This means two people with identical hydroxyzine doses and identical liver metabolism can still have different gut responses because of differences in their intestinal bacteria, an area of research that is still in early stages but helps explain why GI drug side effects are so variable.